Equipment that acquires an image of a subject by collecting radiant energy emitted from an object without the supply of light from the outside is referred to as an infrared (IR) thermal imaging system. Currently, the fields of application of IR thermal imaging systems have been extended to military equipment such as missile guidance systems, sights for personal weapons, and night visions systems of unmanned aerial vehicles that are used to perform aviation disaster prevention and aerial unmanned reconnaissance and surveillance; thermal imaging diagnosis systems, that is, intelligent medical systems that, in the medical field, measure and analyze minute changes in the temperature of the surface of the human body without imposing pain or burden on the human body, output medical information about the presence or degree of a disease and perform the prevention of a disease; unmanned forest fire monitors; environmental surveillance systems for the monitoring of marine pollution or the like; temperature monitoring systems used in semiconductor processing lines; building insulation and leakage finding systems; and electric/electronic Printed Circuit Board (PCB) circuit and part inspection systems; etc., and the demands therefore have increased.
Infrared sensors function to detect the temperature of a target, and may be basically divided into uncooled infrared sensors and cooled infrared sensors. Uncooled infrared sensors are constructed without a cooling device, and operate based on the principle of detecting a change in the characteristic of a material at a room temperature of about 300 K. Among uncooled infrared sensors, micro-bolometers are resistance thermometers used to measure an infrared ray, and use metal or a semiconductor having the property in which when an infrared ray is applied, temperature increases and electric resistance changes as a detection element. Bolometer-type infrared sensors detect a current signal generated by a change in the resistance of a detection element attributable to an infrared ray via a read-out integrated circuit (ROIC), perform the signal processing of the current signal, output the processed current signal, thereby providing the results of temperature detection.
FIG. 1 is a basic configuration diagram of the ROIC 10 of an uncooled infrared sensor.
The ROIC 10 includes a unit cell 14 configured to supply a bias to an active bolometer, a skimming cell 12 configured to generate a skimming current used to eliminate a DC bias current from a signal current by supplying a bias to a blind bolometer, and an integrator 16 configured to integrate signal currents. The unit cell 14 maintains the bias of an active bolometer for detecting an infrared ray, generates an optical current, and performs a cell selection function. The unit cell 14 includes an active bias transistor Tr and a selection switch. The active bias transistor applies an active bias voltage Bolo_Bias-Vt for the operation of the active bolometer. The selection switch performs an ON/OFF operation in response to a control signal Active_Sel input from the outside, thereby controlling connection to a column bus. Accordingly, a current generated by the active bolometer may be transferred to a column circuit through a column bus. The skimming cell 12 maintains the bias voltage of the blind bolometer, generates a skimming current used to eliminate the DC bias current of the active bolometer, and performs a cell selection function. The skimming cell 12 includes a blind bias transistor and a selection switch. The blind bolometer is a resistor whose value is not varied by an infrared ray, and has the same resistance value as the active bolometer. The blind bias transistor applies a bias voltage Vskim−Blind_Bias−Vt for the operation of the blind bolometer. The selection switch performs an ON/OFF operation in response to a control signal Blind_Sel applied from the outside, thereby finally controlling connection to a column bus. Accordingly, the generated current may be transferred to a column circuit through the column bus.
In accordance with this configuration, when an infrared ray enters in the state in which a bias voltage has been applied to an active bolometer, a DC bias current and a signal current attributable to the entrance of the infrared ray flows through the active bolometer. Generally, the DC bias current is considerably higher than the signal current, and thus a change in output voltage attributable to the signal current is made to be considerably small if integration is performed via the integrator 16 without performing any processing. Accordingly, the skimming cell 12 generates a skimming current that is, used to eliminate a DC bias current using a blind bolometer that has approximately the same resistance value as the active bolometer. Only a target signal should be considerably amplified in such a way that a skimming current having the same magnitude as a DC bias current is generated by applying an appropriate bias voltage to the blind bolometer, thereby eliminating the DC bias current flowing through the active bolometer (current skimming) and then transferring only a signal current to the integrator 16. By supplying the amplified target signal to the active bolometer, only the signal current may be made to be applied to the integrator 16.
Meanwhile, in order to acquire a temperature detection result image, micro-bolometers may be configured as a focal plane array (FPA) in a 2D arrangement. Meanwhile, since the resistance of each bolometer is not ideally uniform, the output of the micro-bolometer FPA has non-uniform characteristics. In the meantime, in a process in which in order to extract a signal current, a DC bias current is eliminated and amplified, the non-uniformity between pixels is also amplified in proportion to the amplification of the signal current. As a result, a problem arises in that the saturation of the output signal occurs and an amplification factor is limited.
FIG. 2 is an output graph of the ROIC 10, which illustrates a histogram regarding outputs for respective pixels. The output of the ROIC 10 may fluctuate within a circuit dynamic range. Due to initial non-uniform characteristics for respective pixels, as illustrated in the histogram of FIG. 2, a range corresponding to a non-uniform range occurs, and a portion corresponding to the range cannot be used when a target is viewed. A final scene dynamic range may be defined as a range that is obtained by excluding a non-uniform range from a circuit dynamic range. Accordingly, it is important to ensure a maximum scene dynamic range by reducing a non-uniform range to a minimum range.